The art relating to flexible couplings is replete with means and methods for accommodating the often dramatic loads that are experienced during deep sea drilling operations. Each newly discovered means or method is generally aimed at improving the capability of the drilling equipment to more efficiently and quickly drill. However, in order to be successful the inventor must inevitably deal with the varying forces in the ocean and at the varying and increasing depths in which drilling operations are now being conducted. The improved equipment must also take into account the weight of the drilling equipment and the inevitable increases in this weight caused by the simple addition of length for the ever increasing depth of drilling operations.
Typically, the off-shore drilling equipment that is used in drilling operations is comprised of a number of specialized elements connected in a long riser column each of which must be designed to act in unison with the entire drilling system. To assemble these elements, there is obviously a need for a number of connector elements of varying design and purpose to connect the system elements and form the drilling riser. One form of these connectors is a flexible connector that is designed to prevent damage to the sensitive system elements that are otherwise unable to withstand the dynamic forces in the ocean environment as they interact with the drilling system. Since it can be assumed that the bottom end of the drilling riser of the drilling assembly is anchored to the ocean floor where drilling is to take place, then the forces that act against the drilling riser are generated both by the ocean and by the movement of the drilling vessel, either a drill ship or an off-shore floating platform. The movement of the drilling vessel, which is normally also anchored in place thereby limiting lateral movement, is generally limited to the rise and fall and oscillating rotation, pitch and yaw caused by ocean swells. The rise and fall of the drilling vessel is largely accommodated in the drilling riser by a telescoping member. Since the differential between the highest to lowest positions of the drilling vessel as caused by such ocean swells can be rather large, the telescoping joint is, of necessity, equally large. Consequently, since any lateral movement of the drilling vessel results in a bending force being applied to the telescoping joint, the larger the telescoping joint the larger the resulting bending force. Of course while the telescoping joint could be engineered to accommodate ever larger bending forces, albeit at significant increased cost, practically speaking, a telescoping joint of the size necessary to accomodate the ocean swells cannot be subjected to significant bending forces since the interfacing members of the joint would tend to gall and seize and cause the drilling operation to shut down for repair. The telescoping joint therefore is one of the sensitive system elements that must be protected from the various forces impinging on the drilling riser.
Typically there are two flexible connectors employed to accommodate the dynamic forces acting against the drilling riser--one at the top of the drilling riser where the riser connects to the drilling vessel, and one at the bottom where the riser is connected to the anchoring means. The upper flexible connector normally attaches to the riser a short distance above the telescoping joint and hence any lateral or rotational movement of the drilling vessel will quickly be transmitted to the connector. Also when the rise and fall of the telescoping joint exceeds its design limits and bottoms or tops out, the resulting compression and tension forces are also quickly transmitted to the upper flexible connector. In addition to these compression and tension loads, the upper flexible connector also needs to be able to accommodate the weight of the drilling riser. The tension load generated by the drilling riser is a static load due to its total weight which load is normally balanced by tensioning devices that attach between the drilling vessel and the bottom of the telescoping joint. However when the drilling operation is to be moved or when the drilling riser or a part thereof needs to be repaired, the upper flexible connector needs to be capable of handling and supporting the weight of the drilling riser which, at varying depths down to the 10,000 ft level can reach eight hundred thousand pounds (800,000 lbs) or more.
The bottommost flexible connector by contrast needs to be designed to accomodate the large tension force that is applied to counterbalance the weight of the drilling riser when it is installed and the fairly constant, and relatively large axially directed tension forces that act along the riser. The bottom flexible connector also needs to accommodate the bending forces acting against the drilling riser, albeit at a relatively larger scale than those acting near the top.
One flexible coupling means designed for accommodating such forces is that shown in U.S. Pat. No. 4,076,284 issued in Feb. 1978 to J. T. Herbert, et al entitled "Ocean Floor Riser Connection". While the original intent of this particular flexible device was to accommodate the flex that occurs between the relatively fixed and anchored well pipe and the drilling riser, the same principles could also be used to design a flexible coupling accommodating the forces that apply against the riser near the top, or between the telescoping joint and the off-shore drilling platform. Herbert et al shows a connector comprising a housing 10 attached by flange 12 to the lower portion of the drilling riser and having a lower bore 1Oa that is smaller than an upper bore 10d, a nipple 20 attached to the upper portion of the drilling riser by flange 22 and which extends into housing 10 having a flared pattern 20c, an elastomeric seal 30 that is bonded to portion 20c and a collar 31 that engages seal 30 and that is held in place by retainer ring 32. The flex connector is capable of handling tilt of the upper riser 25 relative to housing 10 but apparently Herbert et al assumed that the connector would need to accommodate only relatively large tilting forces since the dimension of zone 1Og is indicated as changing very little during such tilting. The Herbert et al connector is also able to remain sealed under both tensile and compresive loads: under tensile load nipple portion 20c acts against seal 30 to maintain the seal, and under compression (here assumed to be a higher external pressure) collar 31 acts against seal 30 to maintain the seal. The inventive aspect of Herbert et al, however, is the use of a guide ring structure for supporting the riser in the housing which guide ring is highly perforate to accommodate scouring of particulate matter that might otherwise lodge between seal 30 and the housing.
Another flexible coupling means designed to accomodate such forces is that described in U.S. Pat. No. 4,068,864 issued to J. T. Herbert et al for a Single Element Flexible Connection. The device described in Herbert '864 comprises a housing 10 a nipple 12 disposed within said housing 10 and having a flared end 12b extending radially outwardly such that it is positioned between the interior wall of cylindrical extension 10c of housing 10 and an interiorly disposed lip 1Od. Between lip 1Od and the interior wall of 12b there is an inner rubber boddy 20, and between the exterior wall of end 12b and a ring 10f affixed to extension 1Oc is a flexible elastomeric body 21. Bodies 20 and 21 are provided to accommodate the various faces impinging on connector 10. When the riser, not shown is subjected to tension that face is transmitted to body 21 in compression via flared end 12b. Likewise compression faces are accommodated by body 20. Finally a series of channels 30 are provided in housing 10 to allow pressure equalization into annular wall 10w and to apply internal pipe pressure to body 21.
Both Herbert et al '284 and Herbert et al '864 are capable therefore of handling the various forces that are typically experienced in a drilling riser. However, these designs may not simultaneously be able to accommodate the need for protecting the telescoping joint which is vulnerable to large bending/tilting forces. A flexible connector designed to protect the telescoping joint needs to be capable of tilting under relatively low bending forces. In order to tilt under low bending forces the flexible connector cannot accommodate relatively large tension/compression forces.
A designer might improve the inherent ability of the above-described Herbert et al patents by employing two such connectors arranged in mirror-image relation. This arrangement would enable the designer to accommodate the same axially-directed loads and, since the axially-directed load could be distributed between two connectors in effect, reduce the force required to bend or tilt the double connector by half. U.S. Pat. No. 4,068,868 issued to J. H. Ohrt for a "Flexible Joints for Marine Risers" describes such an arrangement. In Ohrt, joint 10 is connected to the upper riser 11 with flange 15 of upper body 13, and to lower riser 12 by flange 45 of lower body 31. The upper and lower bodies 13, 31 are mirror images of each other each comprising a radially outwardly directed shoulder on which bearing assemblies 47 and 48 are positioned. Bearings 47 and 48 are held in position by bushings 25 and 30 which are attached to housing 28. All tensile forces imposed on the drilling riser column are transmitted to bearings 47 and 48 through the shoulders of bodies 13 and 31. Disposed between bodies 13 and 31 is a seal assembly 50 that prevents leakage from the interior to the exterior of the connector. It is a primary requirement of these flexible connectors to prevent leakage of the highly pressurized drilling mud traversing the riser column. Assembly 50 includes intermediate body 51 and an upper lower seal structure 52 and 54. The seal structures 52 and 54 and bearings 47 and 48 are all similar in construction. They each are laminated structures and include alternating layers of rigid segments and elastomeric segments that are substantially spherical in shape to enable the structures to withstand a greater angular tilt or displacement. However, the laminated structure makes the bearings and seals somewhat vulnerable to tensile forces directed normal to their surface. Ohrt, therefore, requires the bearings and seals to be pre-compressed in order to protect the structures from such potentially damaging tensile loads. The amount of precompression required will depend on the range of forces that are to be accommodated in a particular application. For example, when tensile loads are to be accommodated, bearings 47 and 48 will be precompressed sufficiently to accommodate the desired range of tensile forces and avoid any tensile force being applied against seals 57 and 54.
U.S. Pat. No. 4,173,360 issued to L. A. Bergman for a "Flexible Sealing Joint" is very similar to Ohrt's flexible connector. Bergman includes the concept of two bearings 48a and 48b positioned between flanges (shoulders) 46a and 46b of upper and lower members 44a and 44b and annular flanges (retainers) 40a and 40b. Flanges 40a and 40b are affixed to housing body 38. An intermediate section for sealing between upper and lower members 44a and 44b includes a central element 60, two rigid rings 58a and 58b and flexible elements 62a and 62b disposed therebetween. These several seal and/or bearing structures are similarly subjected to a predetermined level of precompression. The unique aspect of Bergman is that the substantially spherical laminated structure is used only for the bearing elements 48a and 48b (those required to accommodate a load) and that the alternating layers of rigid and then flexible segments are not uniform in thickness but are tapered such that the thicker edges of the several segments are adjacent to the high pressure side of the connector's wall.
The limitations of Ohrt and Berman, described above, are not as great as with the Herbert et al patents in that they can accommodate a larger range of tensile/compression forces and still provide for a relatively low tilting force. However, both Ohrt and Bergman have an inherent limitation in that they must be designed with a specific range of forces that they are to accommodate and generally, as the desired range increases, the bearing structure will be stiffer and therefore the tilting force will likewise be greater. As such, none of the above-described connectors are capable of accommodating the forces that need to be accommodated by the upper flexible connector and at the same time provide significant protection to the telescoping joint. However since they each are readily adaptable to a given range of forces acting on the flexible connector they are perfectly adapted to accommodate the needs of the bottom flexible connector.
Therefore it is an object of the present invention to provide a flexible connector means that is capable of accommodating the various forces that are acting on the upper flexible connector position of a drilling riser.
It is a further object of the present invention to devise a flexible connector means that will provide significant protection to the telescoping joint against damage by bending forces impinging on a drilling riser.
It is a further object of the present invention to provide a flexible connector means that is capable of accommodating two discreetly different load ranges.